Graffiti. I love to photograph graffiti because it's such a transitory artform, but it's often said that it's "photographing someone elses art" and that's a complete valid criticism. I feel the same way about it, and to a lesser extent to architectural photography.

The only response I really have relates to the aforementioned transitory nature of street art. It feels less about, to all intents and purposes, "photocopying" someone else work and passing it off as something creative on your part, but documenting a changing scene.